Towed agricultural implements, e.g., plows and the like, are attached to a tractor using what is known as a 3-point hitch. The hitch, often hydraulically-operated, can be raised and lowered to vary the height of the implement with respect to the surface of the ground.
There are several reasons why it is desirable to vary such height and to repetitively position the implement at a particular height. As an example, when a field is being tilled, it is preferred that the tillage implement engage the ground to the same depth for each "pass" of the implement through the field and to the same depth with respect to the entirety of any particular pass. Planting, time required to till and tractor fuel consumption are all factors which make constant-depth tillage desirable.
And that is not the only reason. A field is prepared to have a tilled portion for crop growing and non-tilled "headlands" at either of two opposed field boundaries. The headlands, covered with weeds, grass or the like, are usually at a somewhat higher elevation than the tilled portion. When the tractor and implement reach the end of a pass at a headland, the implement is raised to "clear" such headland while the tractor is making a U-turn into the next pass.
Since the 3-point hitch raises and lowers relatively slowly, the cumulative time required to repetitively raise and then again lower the implement can have a material effect upon the overall time required to till the field. Therefore, it is desirable to raise the implement to no more than a preselected height, less than its maximum height, to comfortably clear the headland while yet avoiding spending inordinate time in implement raising and lowering.
Still another reason to vary implement height relates to the differing heights used to till and to transport, i.e., move the implement from site to site. When transporting the implement, it is preferred to raise it to its maximum possible height to clear any obstacles in the path of the tractor.
A number of arrangements have been developed to control implement height. Perhaps the earliest was a pivoted lever. Another, more-recent example is disclosed in U.S. Pat. No. 5,261,495 (Szymczak). The Szymczak system is used to control the position of a 3-point hitch and of an agricultural implement connected thereto. The system includes separate, laterally-spaced rotary knobs for selecting, among other parameters, (a) pure position control, draft control or some mix thereof, (b) maximum raised position of the 3-point linkage, and (c) the desired position of the 3-point linkage, presumably at some location below the maximum raised position.
The interface assembly disclosed in U.S. Pat. No. 5,231,892 (Haight) has a turret-like structure in which is fitted a pivot-mounted lever for setting the lowered position of an implement attached to a rockshaft. The lowered position is established by a thumbwheel-positioned abutment which can be defeated (to position the implement at its minimum possible height) by pivoting the lever upwardly over the top of the abutment. When the lever is moved in the raise direction, it can be locked in a transport position by pivoting the lever upwardly over the top of a fixed, projecting tang.
While these prior art mechanisms have apparently been satisfactory for the intended purpose, they are not without some disadvantages. For example, the Haight interface assembly apparently has no provision for setting the raised implement position incrementally below its maximum raised position. And repair of such assembly would seem to be a rather daunting task.
An improved mechanism for controlling implement position which addresses certain shortcomings of the prior art, which is easy to use, which permits an operator to set an intermediate implement position between the corresponding maximum and minimum available positions and which permits tactile operation would be an important advance in the art.